1. Field of the Invention
The present invention relates to a write-once optical disk which can record and regenerating information by illuminating and heating itself, and to an optical recording method using the same.
2. Description of the Related Art
In recent years, as a moving image is digitized, the requirement of increasing recording density of and giving larger capacity to a recording medium is increasing.
In addition, as a substitute for a magnetic storage medium which has been used conventionally, the optical recording medium has been energetically researched and developed which can possess high recording density without bringing a recording head in contact with the optical recording medium. The optical recording medium includes a read-only type which can only read the information, a write-once type which can only record and regenerating the information, and a rewritable type which can record, regenerating and erase the information. Among them, optical recording media on which a user can record information are the write-once type and the rewritable type.
A rewritable type optical recording medium can not only record and regenerating the information, but also re-record the information after having erased it, and accordingly is suitable for use in repeatedly rewriting the information.
On the other hand, a write-once type optical recording medium can record and regenerating information as needed, but cannot erase and re-record the information. The fact is seemingly considered as a disadvantage, but becomes a characteristic of “prevention of falsification for data”, and accordingly has been recognized to be useful for files for distribution, long-period storage, back up or the conservation of an official document.
A write-once optical recording medium includes several types such as CD-R, DVD-R and Photo CD. The medium also includes a type in which a recording layer is composed of an organic coloring material as a main component and the other type in which the recording layer is composed of a metallic element or a non-metallic element as a main component, as known types, and the former is currently a mainstream.
However, an organic material suitable for a laser having a short wavelength has not yet been found, although it is demanded as the density of recorded information increases. Any organic material does not have an optical property equal to a conventional one for a laser with a blue wavelength. This is because although the organic material needs to downsize its molecular skeleton or shorten a conjugating system in order to possess an absorption band around the blue wavelength of the laser, such an organic material then causes the reduction of its absorption coefficient, namely, of its refractive index. In other words, there are many organic materials which have an absorption band around the blue wavelength of the laser and can control the absorption coefficient, but any of them does not have high refractive index and can not provide a high degree of modulation.
For this reason, a write-once type optical recording medium having a recording layer composed of a metallic element as a main component has been variously studied in terms of the design of an inner structure composed of a recording layer and the other layer arranged on the periphery, and of a recording type using the structure. For instance, such methods are known as to open pits in a medium by irradiation with a laser, and as to record information by changing the reflectance of the layer through changing the structure by using a phase change or an alloying technique.
In a pit type recording medium, a study on a medium using a Te film has been progressing. For instance, Japanese Patent Application Laid-Open No. S58-189850 discloses the method of forming an oxide film on the surface of the Te film so as to improve its environment resistance. In addition, a method of adding Se or C to Te similarly to improve the environment resistance is proposed, or a study of a CS2—Te film has been proceeded (for instance, see “Kioku•Kiroku-Gijutsu Handbook (Recording/Storage Technology Handbook)”, Maruzen Co., Ltd., Aug. 25, 1992, pp. 543-546). A method of lowering a melting point by alloying Te with Bi, Zn, Cd, In, Sb, Pb and Sn so as to improve the recording sensitivity of the Te film has also been proposed (for instance, Japanese Patent Application Laid-open No. S58-9234).
A proposed representative phase change type includes a type of using a TeOx film or a TeOx—Pd film (for instance, see “Kioku•Kiroku-Gijutsu Handbook (Recording/Storage Technology Handbook)”, Maruzen Co., Ltd., Aug. 25, 1992, p. 546), and a bilayer type of using Bi2Te3 as a reflective layer and Sb2Se3 as a phase change recording layer (for instance, see “Hikari-Kiroku Gijutsu To Zairyou (Optical Recording Technology And Material)”, CMC Publishing Co., Ltd., October 1985, pp. 94-95).
A proposed alloying type includes a type having a recording layer comprising at least two layers of: a layer formed of at least one element selected from the element group consisting of Ge, Si and Sn; and a layer formed of at least one element selected from the element group consisting of Au, Ag, Al and Cu. When a optical recording medium of such an alloying type is used, the recording layer is irradiated with a laser beam, then the bilayer is alloyed, and the alloyed portion is used for recording (for instance, see Japanese Patent Application Laid-open No. H04-226784).
As a method for alloying between a bilayer, a optical recording medium is further proposed which is composed of a first recording layer mainly containing In formed on a substrate, and a second recording layer containing an element selected from the group consisting of elements in Group 5B or 6B in the periodic table placed thereon (for instance, see Japanese Patent Application Laid-open No. H11-34501).
However, the above described pit type medium has difficulty in obtaining uniform pits thereon when recording density has increased. A phase change type medium has a danger of erasing a recording mark in some cases, because of using a phase change between a crystal state and an amorphous state. In addition, an alloying type medium has had a problem that a contrast of a regenerating signal in the recording mark is low. Furthermore, a recording medium of the alloying type has difficulty in increasing the recording density by a multilayered structure because the recording layer containing a metallic element as a main component used in the medium has a low transmittance.